The following generally relates to integrated circuits. It finds particular application to integrated circuit chip/substrate assemblies. More particularly, it is directed towards mounting integrated circuit chips on a substrate, for example, to maintain chip/substrate gap definition.
An integrated circuit (IC) generally is a relatively thin chip (e.g., silicon) consisting of at least two interconnected semiconductor elements, including active (e.g., transistors, etc.) and/or passive (e.g., resistors, etc.) elements. These elements can be analog or digital. Typically, an integrated circuit chip is classified as analog, digital, or mixed signal, depending on whether the interconnected semiconductor elements are analog, digital, or both analog and digital. Integrated circuits are used to create components such as microprocessors, digital memory, transceivers, etc. and have become ubiquitous in that they are used in computers, communication systems (e.g., cell phones, etc.), appliances (e.g., microwaves, etc.), transportation, manufacturing, etc., which are integral components of modern societies.
Traditionally, integrated circuits were packaged in ceramic flat packs, which provided reliability and compactness. More recent packaging techniques include dual in-line packages (DIPs), initially in ceramic and then in plastic, pin grid arrays (PGAs), leadless chip carriers (LCCs), and surface mount packaging. Surface mount technology (SMT), in general, occupies an area about 30-50% less than an equivalent DIP package, with a typical thickness that is 70% less than the equivalent DIP package. Surface-mount types of packaging include, Small-Outline Integrated Circuit (SOIC), Plastic Leaded Chip Carrier (PLCC), Thin Small-Outline Package (TSOP), Ball grid array (BGA), etc. packages. Various techniques are used to mount integrated circuit chips to a substrate, depending on the chip packaging. Examples of mounting techniques include through-hole (e.g., solder and wire wrap) and surface mount.
Integrated circuit chips can also be mounted upside-down in a flip chip configuration. In general, after cutting the wafer into individual dice, the chip is mounted upside down in or on the substrate and solder reflowed. An underfill process, similar to an encapsulation process, may subsequently be used to cover the sides of the die. StressedMetal technology is a convenient way to fabricate flip-chip interconnects. These interconnects can either be mechanically pressed against a board pad or be soldered to it. However, differential thermal expansion between the integrated circuit silicon and the organic board moves the interconnects around. With mechanical interconnects, mechanical contacts can slide over mating board pads. With soldered interconnects, the soldered contacts rely on in-plane spring compliance to take up run-out. The mechanics of soldered springs are a function of a proper definition of the gap between the flip-chip chip and the board. If this gap is too close, the springs can be stretched out flat with little compliance left. If this gap is too far, some springs may not contact their respective board pad. Thus, there is an unresolved need to control the chip-to-board gap.